Method for isolating nucleic acids

ABSTRACT

The invention relates to a method for isolating nucleic acids from a solution, wherein the nucleic acids are adsorbed on a surface containing SiO2 in the presence of alkali halides and alcohol. The invention also relates to the use of a buffer solution containing alkali halides for isolating nucleic acids on a carrier containing SiO2, in addition to a kit for implementing a method for isolating nucleic acids from a solution.

[0001] The present invention relates to a process for isolating nucleicacids from a solution, in which the nucleic acids are adsorbed onto asurface containing SiO₂.

[0002] The present invention further relates to the use of a buffersolution for isolating nucleic acids on a carrier containing SiO₂ and akit for carrying out a process for isolating nucleic acids from asolution.

[0003] The purification and isolation of nucleic acids on mineralcarriers in the presence of chaotropic salts is well known in theliterature.

[0004] Marko et al. [Analyt. Biochem. 121 (1982) 382] and Vogelstein etal. [Proc. Nat. Acad. Sci. 76 (1979) 615] recognised that if the DNAfrom extracts containing nucleic acid is exposed to high concentrationsof sodium iodide or sodium perchlorate, only the DNA binds tomechanically finely divided glass scintillation tubes and comminutedglass fibre membranes or glass fibre plates, whereas RNA and proteins donot bind. The DNA thus bound can be eluted with water, if desired.

[0005] EP 0389063 B1 relates to a process for isolating nucleic acidsfrom a biological source. According to this method the biologicalsources containing DNA, such as blood, cells, plasma, etc., are lysed inthe presence of chaotropic salts in high concentrations and then thenucleic acids are bound to a silica surface. They are then washed andeluted.

[0006] The process described in U.S. Pat. No. 5,155,018 describes theisolation of RNA from biological sources which contain DNA and otheringredients in addition to RNA.

[0007] The biological sample is acidified and mixed with a chaotropicagent such as a guanidinium salt. Silicate particles are added to thesample and under the conditions specified RNA binds to the silicateparticles. Then, again, the RNA is also separated from the particles.

[0008] In WO 95/01359 Colpan et al disclose a process for purifying andseparating nucleic acid mixtures by adsorbing the nucleic acid from analcohol-containing solution with a high ionic strength. The adsorptionsolution contains, in addition to alcohol in a concentration of 1 to 50vol. %, salts in a concentration of 1 to 10 M, of which the chaotropicsalts such as guanidinium thiocyanate, sodium perchiorate or guanidiniumhydrochloride are preferred.

[0009] WO 95/21849 relates to a process for separating double- and/orsingle-stranded nucleic acids from sources which contain these nucleicacids. In this process, too, the nucleic acids are adsorbed on mineralcarriers under conditions which allow binding of the desired type ofnucleic acid, while the unwanted type of nucleic acid does not bind tothis mineral carrier.

[0010] In order to bind predominantly single-stranded nucleic acid to amineral carrier and thus separate it from double-stranded nucleic acid,the treatment conditions are adjusted with an aqueous mixture of salts,particularly chaotropic salts, and alcohol, in accordance with thesamples containing the two types of nucleic acid. The double-strandednucleic acid which is not adsorbed can then be further purified orisolated by known methods.

[0011] Nucleic acids which are used for molecular-biologicalapplications, such as PCR, sequencing and gene transfer, particularlyfor transfection or vaccination, are subject to extremely stringentrequirements with regard to purity and integrity. The use of nucleicacids in molecular diagnostics or molecular medicine presupposes thatthey are free from toxic substances which may lead, for example, topathogenic effects in the organisms which are to be treated.

[0012] One factor which is common to the processes known from the priorart is that chaotropic salts are used in high concentrations forisolating nucleic acids on silica surfaces. Chaotropic substances suchas guanidinium hydrochloride, guanidinium thiocyanate or sodiumperchlorate are highly toxic substances.

[0013] The possibility that the nucleic acids isolated in the presenceof these substances will be contaminated therewith and thus madeunusable or largely unusable for use in molecular biologicalapplications cannot be ruled out.

[0014] The handling of chaotropic substances also constitutes a majorrisk to the health of the user, which means that certain safetyprecautions have to be taken when handling these substances.

[0015] The technical problem on which the present invention is based isto provide an improved process which overcomes the disadvantages knownfrom the prior art. The substances used in this process for binding thenucleic acid should not be toxic. At the same time the process should beas low-cost as possible, for example by using cheap chemicals, and theisolated nucleic acids should be isolated in a quantitatively andqualitatively pure form.

[0016] Surprisingly, the present invention provides a process forisolating nucleic acids from a solution, which solves this problem. Theinvention resides in the fact that in a first step the binding ofnucleic acids to surfaces containing SiO₂ is carried out in the presenceof alkali metal halides in a concentration of 0.1 to 3 M, preferably0.25-1.5 M, and alcohol in a concentration of 37 to 70 vol. %. Thenucleic acids adsorbed onto the surface containing SiO₂ are thenoptionally washed with an alcohol-containing washing buffer and thenucleic acid is eluted with an aqueous salt solution or with water.

[0017] To bind the nucleic acids onto the surface containing SiO₂,aqueous adsorption solutions are used which contain alkali metal halidessuch as NaCl, KCl and LiCl in a concentration of 0.1 to 3 M, preferably0.25-1.5 M, more preferably 0.5-1.25 M and particularly 0.5-1.0 M.Alkali metal halides are non-toxic substances and the handling of thesalt solutions in the concentration used is perfectly safe in terms ofhealth.

[0018] The aqueous adsorption solutions contain, in addition to theabovementioned salts, lower aliphatic, branched or unbranched alcoholswith a chain length of 1 to 5 carbon atoms. The aliphatic alcoholscontained in the solution are preferably methanol, ethanol, propanol,isopropanol and butanol in a concentration of 37-70 vol. %, preferably37-50 vol. %. Of the abovementioned alcohols, ethanol and/or isopropanolin a concentration of 37-70 vol. % are particularly preferred.

[0019] Surfaces containing SiO₂ may be for example porous or non-poroussilicon oxides or metal-silicon mixed oxides, silica gels, materialsbased on glass, e.g. modified or unmodified glass particles or groundglass, quartz, zeolites or mixtures of one or more of the abovementionedsubstances.

[0020] By a surface is meant, for the purposes of the present invention,any microporous boundary layer.

[0021] In a particularly preferred embodiment of the process accordingto the invention, the surface containing SiO₂ is a porous membrane or afilter made of silica gel, glass fibres or quartz fibres.

[0022] In another embodiment of the process according to the invention,the term surface in the wider sense also includes a layer of particlesor granules or fibres, such as e.g. silica gel fleece.

[0023] The bound nucleic acid may be eluted according to the inventionusing water or aqueous saline solutions as eluant. The saline solutionsused are buffer solutions known from the prior art, such as, forexample, morpholinopropanesulphonic acid (MOPS),tris(hydroxymethyl)aminomethane (TRIS),2-[4-(2-hydroxyethyl)-1-piperazino]ethanesulphonic acid (HEPES) in aconcentration of 0.001 to 0.5 mol/litre, preferably 0.01 to 0.2mol/litre, most preferably 0.01 to 0.05 molar solutions.

[0024] In another embodiment of the process according to the invention,the nucleic acids contained in the eluate may preferably be isolated byalcoholic precipitation.

[0025] The nucleic acids isolated by this process are free from toxicsubstances and are thus suitable for use in molecular biology.

[0026] The term “nucleic acid” should hereinafter be understood in itswidest sense, i.e. to include ribonucleic acids (RNA) and alsodeoxyribonucleic acids (DNA) in all lengths and configurations, such asdouble-stranded, single-stranded, circular and linear, branched, etc.,and all possible subunits thereof, such as e.g. monomeric nucleotides,oligomers, plasmids, viral and bacterial DNA and RNA, as well as genomicand non-genomic DNA and RNA from animal and plant cells or othereukaryotes, mRNA in processed and unprocessed form, tRNA, hn-RNA, rRNA,cDNA as well as all other conceivable nucleic acids.

[0027] The process according to the invention makes it possible toisolate nucleic acids of every origin from solutions. The samplecontaining nucleic acids originates, for example, from animal or planttissues, tissue or cell cultures, bone marrow, human and animal bodyfluids such as blood, serum, plasma, urine, sperm, cerebrospinal fluid,sputum and smears, plants, parts of plants and plant extracts, e.g.juices, fungi, procaryotic or eucaryotic microorganisms such as bacteriaor yeasts, fossilised or mummified samples, soil samples, clarifiedsludge, waste water and foodstuffs (particularly processed, i.e.industrially prepared foodstuffs). Nucleic acids formed by chemicalreactions, e.g. those obtained by polymerase chain reaction (PCR) orplasmid-DNA, genomic DNA and RNA and/or nucleic acids which originatefrom microorganisms may also be isolated according to the invention.

[0028] The process according to the invention is particularly suitablefor isolating plasmid DNA from bacteria, such as e.g. E. coli forsubsequent cloning, transfection or sequencing.

[0029] The lysing of the bacteria is effected using known lysing methodssuch as, for example, alkaline lysing according to Bimboim and Doly(1979) or lysing by heating according to Holmes and Quigley.

[0030] The cell debris as well as the precipitated proteins and thegenomic DNA are eliminated from the viscous lysate by centrifuging orfiltering and a clarified lysate is obtained which contains the plasmidDNA. The plasmid DNA can be purified by ion exchange chromatography, forexample, and the plasmid DNA thus pre-purified can then be isolatedusing the process according to the invention.

[0031] The invention further relates to a kit for isolating nucleicacids from a solution, comprising

[0032] a) an adsorption solution containing 0.25-1.5 M NaCl, KCl or amixture thereof and ethanol or isopropanol in a concentration of 37-70vol. %, and

[0033] b) a surface containing SiO₂.

[0034] The surface containing SiO₂ may be a porous membrane or a filtermade of silica gel, glass fibres or quartz fibres and may be arranged ina suitable apparatus. The kit preferably additionally contains solutionswhich are suitable for lysing, as well as washing and elution buffers asdescribed above.

[0035] The nucleic acids isolated according to the invention are freefrom enzymes that break down nucleic acids and are thereforesufficiently pure that they can immediately be further treated andprocessed in various ways.

[0036] The nucleic acids produced according to the invention may be usedfor cloning and act as substrates for all kinds of enzymes, such as forexample DNA polymerases, DNA restriction enzymes, DNA ligase and reversetranscriptase.

[0037] The nucleic acids prepared by the process according to theinvention are particularly suitable for amplification, especially PCR,Strand Displacement Amplification, the Rolling Circle process, LigaseChain Reaction (LCR) and similar processes.

[0038] The process according to the invention is also particularlysuitable for preparing nucleic acids for use in diagnostics,particularly for a method of diagnosis which is characterised in thatthe nucleic acid purified by the process according to the invention isamplified in a subsequent step and then and/or at the same time thenucleic acid thus amplified is detected (e.g. Holland, P. M. et al.,1991, Proc. Natl. Acad. Sci. 88, 7276-7280. Livak, K. J. et al., 1995.PCR Methods Applic. 4, 357-362; Kievits, T. et al.., 1991. J. Virol.Meth. 35, 273-286; Uyttendaele, M. et al., 1994. J. Appl. Bacteriol. 77,694-701).

EXAMPLE 1

[0039] Isolation of Plasmid DNA in the Presence of NaCl and Alcohol inVarious Concentrations

[0040] 390 μl aliquots of the individual buffers (0.25-1.5 M NaCl; 50 mMTris, pH 8.5; 15% (w/v) isopropanol) were mixed with 1-μg of plasmid DNA(pCMVβ; Messrs Clontech #6177-1) (c=1 μg/μl) and combined with variousamounts of isopropanol, corresponding to a total amount of 28.9-49.8 vol% of isopropanol in the binding buffers in question. After five minutes'incubation at ambient temperature (20-25° C.) the mixtures weretransferred into a column containing a silica membrane and passedthrough the silica membrane in vacuo (about 600 mbar; using the QIAvac6S of Messrs QIAGEN GmbH). Then it was washed with 750 μl of PE buffer(10 mM Tris, pH 7.5; 80% ethanol) and air was passed through themembrane until it dried. Elution was carried out by the addition of 100μl of EB buffer (10 mM Tris, pH 8.5) and the yield was determinedphotometrically at 260 nm. The results are shown in Table 1. TABLE 1Isopropanol (total) C_(NaCl) 0.25 M 0.5 M 0.75 M 1.0 M 1.25 M 1.5 M 28.9vol % DNA 4.7[μg] 5.8[μg] 8.0[μg] 7.3[μg] 7.6[μg] — OD₂₆₀ 0.0934 0.11540.1593 0.1456 0.1522 34.3 vol % DNA 8.2[μg] 8.1[μg] 7.4[μg] 7.5[μg]7.0[μg] 6.4[μg] OD₂₆₀ 0.1644 0.1611 0.1484 0.1491 0.1390 0.1493 39.0 vol% DNA 6.2 g[μg] 5.8[μg] 8.0[μg] 6.2[μg] 7.3[μg] 5.8[μg] OD₂₆₀ 0.12450.1161 0.1603 0.1244 0.1452 0.1211 43.1 vol % DNA 7.2[μg] 7.7[μg]6.2[μg] 6.9[μg] 7.4[μg] 5.5[μg] OD₂₆₀ 0.1444 0.1534 0.1231 0.1375 0.14690.1100 46.6 vol % DNA 5.7[μg] 8.4[μg] 7.3[μg] 8.5[μg] 7.9[μg] 5.6[μg]OD₂₆₀ 0.1129 0.1669 0.1454 0.1701 0.1585 0.1104 49.8 vol % DNA 6.5[μg]6.0[μg] 6.1[μg] 6.3[μg] 6.3[μg] 3.3[μg] OD₂₆₀ 0.1294 0.1200 0.12210.1252 0.1258 0.0719

EXAMPLE 2

[0041] Isolation of Plasmid DNA at Different Alcohol Concentrations

[0042] 500 μg (c=1 μg/μl) of plasmid DNA (pCMVβ; Messrs Clontech#6177-1) were dissolved in 5 ml of Q1 buffer (1.25 M NaCl; 50 mM Tris,pH 8.5; 15 vol % isopropanol). Then isopropanol was added in an amountcorresponding to a total content of isopropanol in the binding buffer of22.7-43.3 vol %, carefully mixed and incubated for 5 min at roomtemperature (20-25° C.). The DNA/Q1/isopropanol mixture was pressed intoa glass fibre filter (Messrs Sartorius, Minisart Series) and themembranes were blown dry. Elution was carried out with 1 ml of TE buffer(10 mM Tris, pH 7.5; 1 mM EDTA). The yield was determinedphotometrically at 260 nm. The results are shown in Table 2. TABLE 2Isopropanol (total) DNA [μg] DNA [%] OD₂₆₀ values 22.7 vol % 242 480.2419 29.2 vol % 416 83 0.4159 34.6 vol % 383 77 0.3826 39.3 vol % 41583 0.4146 43.3 vol % 412 82 0.4118

EXAMPLE 3

[0043] Isolation of Plasmid DNA in the Presence of KCl and Alcohol inVarious Concentrations

[0044] 390 μl aliquots of the individual buffers (0.25-1.0 M KCl; 50 mMTris, pH 8.5; 15% (w/v) isopropanol) were mixed with 10 μg of plasmidDNA (pCMVβ;

[0045] Messrs Clontech #6177-1) (c=1 μg/μl) and combined with variousamounts of isopropanol, corresponding to an amount of up to 57.3 vol %of isopropanol (see Table 3) or up to 71.4 vol % of isopropanol togetherwith ethanol in the binding buffers in question. After five minutes'incubation at ambient temperature (20-25° C.) the mixtures weretransferred into a column containing a silica membrane (QIAquick made byMessrs QIAGEN GmbH, #28104) and passed through the silica membrane invacuo (about 600 mbar; using QIAvac 6S of Messrs QIAGEN GmbH). Then itwas washed with 750 μl of PE buffer (10 mM Tris, pH 7.5; 80% ethanol)and air was passed through the membrane until it dried. The mixture waseluted by the addition of 100 μl of EB buffer (10 mM Tris, pH 8.5) andthe yield was determined photometrically at 260 nm. TABLE 3 IsopropanolDNA yield ethanol/ DNA yield C_(KCl) (total) [μg] OD₂₆₀ isopropanol [μg]OD₂₆₀ 0.25 M 46.6 vol % 9.5 0.1185 66.7/5.9 vol %  9.7 0.1208 0.5 M 49.8vol % 8.8 0.1101 71.4/4.2 vol % 10.0 0.1355 0.75 M 46.6 vol % 8.8 0.109860.0/5.9 vol % 10.0 0.1322 0.75 M 57.3 vol % 9.6 0.1194 66.7/4.2 vol %10.0 0.1770 1.0 M 43.1 vol % 10.0  0.1727 33.3/9.8 vol % 10.0 0.1280

EXAMPLE 4

[0046] Isolation of Plasmid DNA in the Presence of LiCl; NaCl or KCl andAlcohol in Various Concentrations

[0047] 390 μl aliquots of the individual buffers (0.25-1.0 M salt; 50 mMTris, pH 8.5; 15% (w/v) isopropanol) were mixed with 10 μl (≈10 μg) ofplasmid DNA (pCMVβ; Messrs Clontech #6177-1) (c=1 μg/μl) and combinedwith various amounts of ethanol, corresponding to an amount of up to71.4 vol % of ethanol (see Table 4 below) in the binding buffers inquestion. After five minutes' incubation at ambient temperature (20-25°C.) the mixtures were transferred into a column containing a silicamembrane and passed through the silica membrane in vacuo (about 600mbar; using QIAvac 6S of Messrs QIAGEN GmbH). Then it was washed with750 μl of PE buffer (10 mM Tris, pH 7.5; 80% ethanol) and air was passedthrough the membrane until it dried. The mixture was eluted by theaddition of 100 μl of EB buffer (10 mM Tris, pH 8.5) and the yield wasdetermined photometrically at 260 nm. TABLE 4 LiCl NaCl DNA DNA KCl[Salt]/[EtOH] [μg] OD₂₆₀ [μg] OD₂₆₀ [μg] OD₂₆₀ 0.25 M/66.7 vol % 7.20.0721 7.6 0.0763 10.0 0.1087 0.5 M/71.4 vol % 7.9 0.0794 7.8 0.078310.0 0.1005 0.75 M/60 vol % 9.0 0.0895 8.6 0.0862  8.8 0.0875 0.75M/66.7 vol % 9.0 0.0857 7.7 0.0771 10.0 0.1001 1.0 M/33.3 vol % 9.60.0963 10.0  0.1253  8.9 0.0890

EXAMPLE 5

[0048] Isolation of Larger Amounts of DNA

[0049] Increasing amounts of plasmid DNA (c=1 μg/μl; pCMVβ; MessrsClontech #6177-1) were dissolved in 15 ml of Q1 buffer (1.25 M NaCl; 50mM Tris, pH 8.5; 15 vol % isopropanol). Then isopropanol was added togive a final concentration of 49.8 vol %, the ingredients were carefullymixed and incubated for 5 minutes on the laboratory bench.

[0050] The DNA/Q1/isopropanol mixture was transferred into a 20 mlsyringe fitted with a syringe pre-filter containing a silica membrane(Messrs Sartorius, Minisart Series). The mixture was forced through thefilter under uniform pressure, the filter was removed and air was blownthrough the membrane again to remove the alcohol residues. Elution wascarried out by forcing 5 ml of TE buffer (10 mM Tris, pH 7.5; 1 mM EDTA)through the syringe pre-filter using a fresh 5 ml syringe. The yield wasdetermined photometrically at 260 nm. The results are shown in Table 5.TABLE 5 pCMVb [mg] 1.5 2.0 2.5 3.0 DNA yield 1.274 1.577 2.367 2.523[mg] DNA yield 85 79 95 84 [%] OD₂₆₀ 0.2547 0.3153 0.4733 0.5046

[0051] Results:

[0052] The recovery rate of the DNA is between 80 and 90% even withlarger quantities of plasmid DNA.

EXAMPLE 6

[0053] Isolation of Plasmid DNA in the Presence of VariousConcentrations of NaCl and Alcohol

[0054] 340 μl aliquots of an NaCl solution (0 M; 0.1 M; 0.25M; 0.5 M;1M; 2.5 M; 5 M; saturated solution; 50 mM Tris, pH 8.5) were mixed with10 μl of a solution containing 1 μg/μl plasmid DNA (pCMVβ; MessrsClontech #6177-1). 350 μl of isopropanol were added to each sample,resulting in a concentration of 50 vol. % for each mixture. After fiveminutes' incubation at ambient temperature (20-25° C.) the mixture wastransferred into a column (QIAvac 6S; QIAGEN GmbH) containing a glassfibre/silica membrane (QIAprep 8-well strips; QIAGEN GmbH). TheDNA-containing solutions were passed through the membrane in vacuo(about 600 mbar) and then eluted with 200 μl of EB buffer (10 mM Tris,pH 8.5). The yields of DNA (as a percentage of the amount put in) in thequadruple measurements taken at each salt concentration as well as theaverage are shown in Table 6 below. TABLE 6 Yields [%]: value 1 valuevalue value average STDEV 0 M  17  16  19  18 17.5 1.291 0.1 M 100 100100 100 100 0.000 0.25 M 100 100 100 100 100 0.000 0.5 M 100 100 100 100100 0.000 1 M 100 100 100 100 100 0.000 2.5 M 100 100 100 100 100 0.0005 M  70  73  74  73 72.5 1.732 saturated  83  68  75  79 76.3 6.397

[0055] The result of this test shows that in the recovery of DNA theyield is reduced at very high salt concentrations (5 M, saturated) bycomparison with lower salt concentrations of between 0.1 and 2.5 M.

EXAMPLE 7

[0056] Isolation of Plasmid DNA in the Presence of Various Salts orVarious Alcohol Concentrations

[0057] 500 μg aliquots of plasmid DNA were dissolved in 5 ml of TEbuffer (10 mM Tris-Cl; pH 8.0; 1 mM EDTA; QIAGEN GmbH) and isopropanolwas added until the final concentrations of alcohol in the individualmixtures were 41.2, 50, 66.7 and 75 vol-%. All the measurements for theDNA isolation were carried out with a QIAprecipitator Maxi (QIAGEN GmbH)in a QIAvec 6S column (QIAGEN GmbH). Table 7 shows the results of thetests, in which the salt concentration in the different mixtures was1.25 M NaCl. The tests were each carried out several times, and Table 7shows the results for the individual samples and the average withstandard deviation calculated therefrom. TABLE 7 Yields [%]: Vol %isopropanol 41.2 50 66.7 75 1 81.1 100 60.6 37.3 2 93.1 93.2 69.9 42.9 367.7 77.2 86.4 75.5 4 55.2 70.5 88.4 67.1 5 71.8 68 63 6 51.4 65 60.6averages [%] 74.3 77.4 73.1 57.7 STDEV 16.412 15.892 10.560 13.397

[0058] Then the experiments described above were modified by changingthe nature of the salt solution used: the 1.25 M NaCl solution wasreplaced by a 1.25 M KCl solution. The results of the experiments withKCl solution are shown in table 8. TABLE 8 Yields [%]: Vol % isopropanol41.2 50 66.7 75 1 70.9 82.9 76.4 27 2 69 79.8 66.8 37 3 66.1 75.1 79.084.7 4 68.9 72.6 76.7 65.4 5 68.8 63.8 44.8 6 70.6 67.6 55.2 averages[%] 68.7 75.0 71.7 52.4 STDEV 1.977 4.984 5.817 18.975

[0059] Finally, the influence of the buffer on the yield of plasmid DNAwas also investigated. The method described above was used, and in themixture containing a 1.25 M NaCl solution the TE buffer was replaced bya QF buffer (50 mM MOPS; pH 17.0; 15% Isopropanol). The results of themultiple measurements together with the average and standard deviationare shown in Table 9. The difference in the alcohol concentrations inthe individual mixtures from those in Tables 7 and 8 is due to the 15%isopropanol content of the QF buffer. TABLE 9 Yields [%]: Vol %isopropanol 22.7 50 57.5 78.8 1 59.6 79.8 71.7 39.2 2 40.7 78 82.2 39 324.4 91.8 76.8 69.1 4 25.6 57 79.8 67.8 5 72.2 68.4 6 49.2 55.6 averages[%] 37.6 76.7 72.0 56.5 STDEV 16.450 14.461 10.862 13.130

[0060] The results show that the nature of the salt used (NaCl/KCl) hasonly a slight effect on the yield in the isolation of DNA. Similarly,replacement of the buffer has virtually no effect on the yield of DNA(77.4% for 1.25 M NaCl and TE buffer and 75.0% for 1.25 M NaCl and QFbuffer).

1. Process for isolating nucleic acids from a solution, comprising thefollowing steps: a) adsorbing the nucleic acids contained in thesolution on a surface containing SiO₂ in the presence of alkali metaland/or alkaline earth metal salts, b) optionally washing the nucleicacids adsorbed onto the surface containing SiO₂ with analcohol-containing washing buffer, and c) eluting the nucleic acids withan aqueous solution and optionally isolating the nucleic acids,characterised in that in step a) the adsorption of the nucleic acidsonto the surface containing SiO₂ is carried out in the presence of 0.1to 3 M alkali metal and/or alkaline earth metal salts and 37 to 70 vol %of an aliphatic alcohol;
 2. Process according to claim 1, characterisedin that the nucleic acid is plasmid DNA.
 3. Process according to one ofclaims 1 or 2, characterised in that the surface containing SiO₂consists of silica gel, glass fibres or quartz fibres.
 4. Processaccording to one of claims 1 to 3, characterised in that the surfacecontaining SiO₂ is a membrane or a filter.
 5. Process according to oneof claims 1 to 4, characterised in that the alkali metal salts containedin the solution are halides, preferably NaCl and/or KCl.
 6. Processaccording to one of claims 1 to 5, characterised in that in step a) NaClis present in the solution in a concentration of from 0.25 to 1.5 M,preferably from 0.5 to 1.25 M and most preferably from 0.5 to 1.0 M. 7.Process according to one of claims 1 to 6, characterised in that thealcohols contained in the solution are lower aliphatic, branched orunbranched alcohols with a chain length of 1 to 5 carbon atoms. 8.Process according to claim 7, characterised in that the alcoholcontained in the solution is ethanol and/or isopropanol.
 9. Processaccording to one of claims 1 to 8, characterised in that the eluantsolutions used in step c) contain morpholinopropanesulphonic acid(MOPS), tris(hydroxymethyl)aminomethane (TRIS) or2-[4-(2-hydroxyethyl)-1-piperazino]ethanesulphonic acid (HEPES) in aconcentration of 0.001 to 0.5 mol/litre, preferably 0.001 to 0.2mol/litre, most preferably 0.01 to 0.05 mol/litre.
 10. Use of a solutioncontaining 0.25-1.5 M NaCl, KCl or a mixture thereof and ethanol orisopropanol in a concentration of 37-70 vol. % for the adsorption ofnucleic acid onto a surface containing SiO₂.
 11. Kit for carrying outthe process according to one of claims 1 to 9, containing a) a solutionaccording to claim 10 and b) a surface containing SiO₂.